The present invention relates to an image density detecting unit for an image formation apparatus, which detects the amount of light reflected by a document to obtain a desired copying result in accordance with the detection result.
The quality of an image copied onto a copying paper sheet in an image formation apparatus such as an electronic copying machine has recently been improved. This is because the image carrier (to be referred to as a photosensitive drum hereinafter) and the developer (to be referred to as toner hereinafter) have been improved. Furthermore, an electrical improvement based on studies of the bias voltage for a developing unit has been made. Among the various types of improvements, the automatic exposure unit of an electronic copying machine has received great attention and provides a self-detecting function for adjusting the document density. In principle, the density of the document to be copied can be detected by any detecting method. An image which has a proper density can be obtained in accordance with the above detecting operation in cooperation with an increase or decrease in illuminance (to be referred to as exposure or exposure amount hereinafter), an increase or decrease in the voltage applied to the photosensitive drum, and an increase or decrease in the bias voltage applied to the developing unit. Thus, an optimal copying result may be obtained.
There are two conventional methods for detecting the document density. In one method the exposure is measured at an arbitrary position in the optical path of the optical system for focusing the document image on the surface of the photosensitive drum, to form an electrostatic latent image. In the other method the light-emitting unit for emitting light to the document and the focusing unit for detecting the reflected light are incorporated in addition to the optical system for forming the electrostatic latent image. The former method is realized by the arrangement shown in FIG. 1. Light from a light source 1 is emitted onto a document (not shown) placed between a document table 3 and a document cover 5. Light reflected by the document reaches a photosensitive drum 13 through a reflecting plate 7, a lens 9, and a reflecting plate 11, and an electrostatic latent image is formed on the photosensitive drum 13. The document density is detected by a detecting element 15 which is arranged in the optical path.
However, according to the first system described above, since the detecting element 15 uses part of the optical path, the amount of light which forms the electrostatic image is decreased. Furthermore, although the amount of light which is detected by the detecting element 15 corresponds to the amount of light which forms the image on the photosensitive drum, the amount of light emitted from the light source 1 cannot be spontaneously controlled due to the delay time of the electronic circuit. Since the detecting element 15 is disposed in the optical path of the lens 9, the mounting position of the detecting element 15 greatly affects the precision of measurement of the amount of light.
However, according to the second system described above, as shown in FIG. 2, the apparatus has a lens system 22, an optical path 17 for forming an electrostatic latent image on the surface of the photosensitive drum 13 and an optical path 19 for detecting light reflected by the document. As shown in FIGS. 3A, 4 and 5, focusing units 23, 29, and 31 are respectively disposed in a space 21 of the apparatus. Referring to FIG. 3A, the focusing unit 23 is arranged to converge light from a light-receiving plane 25 to the detecting element 15 utilizing regular and irregular reflection by the reflecting plane. When the focusing unit 23 is arranged in the space 21 of the image formation apparatus, the amount of light emitted from the light source 1 and reflected by the document is insufficient, and proper detection can hardly be performed. As shown in FIG. 3B, the light-receiving plane sensitivity distribution or the light distribution is not uniform, so that the average amount of light incident on the light-receiving plane 25 cannot be detected by the detecting element 15. Referring to FIG. 4, the focusing unit 29 may be made of a self-converging lens (Selfoc lens: trademarks) or an assembly of light-transmitting fibers 33 such as optical fibers. Detecting elements 37 and 39 are respectively mounted on the light-converging plane which is opposite to a light-receiving plane 35. However, in the focusing unit of this type, since the area of the light-receiving plane 35 is the same as that of the light-converging plane, the area of the detecting element must be increased, or a plurality of detecting elements must be used, resulting in a high cost. Furthermore, the focusing unit which comprises an assembly of converging light transmitting fibers is expensive. Referring to FIG. 5, the focusing unit 31 is made of an assembly of optical fibers and has a light-receiving plane 41 whose section has a different shape from that of a detecting plane 43. Even if a low-cost optical fiber is used, the shape of the outer structure is complex, which prevents mass production and results in high cost. Furthermore, since the light-receiving plane 41 is wide and the optical fibers must be concentrated at a single point to form the detecting plane 43, the focusing unit becomes large in size because the optical fiber has a maximum allowable curvature.
As described above, the focusing units for detecting the document density have both economic and performance problems. This is especially so in the case of the focusing unit 23 as shown in FIG. 3A, where a sufficient amount of light cannot be obtained and the light-receiving plane sensitivity distribution becomes nonuniform as shown in FIG. 3B. As a result, a proper density of the document cannot be detected.